2 – 8
Introduction
2.2.2.4
AC versus DC coupling
The signal input can be AC or DC coupled. The AC coupling high
pass filter passes signals above 160 mHz and attenuates signals at
lower frequencies. Internal gain stages within the signal path are
always AC coupled, so any user DC o
ff
set will not a
ff
ect the results
at the phase-sensitive detector output (where they would otherwise
generate a “1
×
f” ripple at the output). However, if the input signal
has a significant DC o
ff
set relative to the AC amplitude, then AC
coupling will significantly improve the overall dynamic reserve by
blocking the DC input before it could saturate the preamplifier.
When operating in di
ff
erential mode (A–B), AC coupling may reduce
the SR124’s e
ff
ective common-mode rejection and gain accuracy, as
the blocking capacitors for the AC coupling are only matched to
5% tolerance. This e
ff
ect is most pronounced at frequencies below
10 kHz.
2.2.2.5
Dynamic reserve
The total signal gain for the SR124 is distributed between the AC
signal path ahead of the PSD, and the DC signal path following the
PSD. The product of all gains, AC and DC, combine to provide the
selected full-scale Sensitivity. How this allocation is made between
AC and DC gain determines the dynamic reserve of the lock-in.
Dynamic reserve is traditionally defined as the ratio of the largest
interfering signal that can be tolerated, to the full scale input signal,
expressed in dB. For example, if the SR124 is operating at full scale
sensitivity of 1
µ
V, and an interfering signal of up to 1 mV can be
rejected before overloading, the dynamic reserve is 60 dB.
Many of the noise advantages of a lock-in amplifier come from the
improved noise and stability properties of electronics operating at AC
frequencies compared to DC. Overall total gain is determined by the
full-scale sensitivity, but the relative allocation of that gain between
the AC and DC portions of the instrument rely on a “policy” decision
that the user can influence using the Reserve setting.
For the lowest noise and greatest output stability, set the Reserve
mode to
Low Noise
. This will allocate the maximum gain to the AC
portion of the signal path, and apply the minimum DC gain needed
for the final sensitivity. This configuration minimizes the impact of
o
ff
sets and drift from the DC circuitry, but also tends to reduce the
dynamic reserve since more of the signal gain is applied to the AC
path, before the PSD can act to select the signal of interest.
For the greatest dynamic reserve, set the Reserve mode to
High Res.
This will allocate the maximum gain to the DC portion of the signal
SR124
Analog Lock-In Amplifier
